In a conventional automotive vehicle, steering is accomplished by adjusting the orientation of the front road wheels. The road wheels pivot, either left or right, between stops that mechanically limit the orientation of the road wheels to assure operation within a safe turning radius. For example, the stops may be positioned to allow the wheels to turn within an angle of plus or minus 30° relative to a straight direction of travel. In a conventional steering system, the steering wheel is mechanically coupled to the road wheels through a rack and pinion mechanism wherein rotation of the steering wheel by the driver rotates a pinion to laterally displace a rack to change the orientation of the road wheels. When the road wheels engage a stop, the steering wheel is prevented from further rotation because of the mechanical coupling, thereby alerting the driver. Moreover, conditions may arise that prevent turning of the road wheels even though the mechanical stop has not been engaged, for example, when the road wheel engage a curb.
Steer-by-wire systems have been developed that include an electric motor that drives the pinion to displace the rack and change the road wheel orientation. U.S. Pat. No. 6,580,989, issued Jun. 17, 2003 to Husain et al., describes a steer-by-wire system that includes a rack and pinion mechanism operated by an electric motor. Steering commands are inputted by the operator using a steering wheel similar to mechanical steering systems. Electrical sensors detect rotation of the steering wheel and provide a signal to a controller that actuates the electric motor to reorient the road wheels.
In a mechanical steering system, the driver experiences a steering feel as a result of the mechanical linkage between the road wheels and the steering wheel. It is desired to provide the driver with a similar steering feel in steer-by-wire systems. For this purpose, it is proposed to couple the steering column to a reaction torque generator. A preferred reaction torque generator comprises an electric motor coupled to the steering column through a belt and pulley mechanism. In response to rotation of the steering wheel to input a steering command, the controller actuates the electric motor to apply an opposite torque to the steering column and thereby provide a feeling of resistance to the driver. The degree of resistance is determined by the controller based upon applicable steering parameters including vehicle speed, steering wheel angle, steering wheel turn rate, yaw rate, rack load, and lateral acceleration, and is intended to provide a feel similar to mechanical steering systems. The motor is sized to generate torque to produce the desired steering feel, but does not provide sufficient torque to lock the steering column, so is not able to alert the driver when the road wheels have engaged a stop or are otherwise prevented from turning.
It is known to equip a steering column with a mechanical stop that prevents endless spinning of the steering wheel, but is unrelated to the position of the road wheels. One advantage of steer-by-wire systems is that the steering ratio, by which is meant the angular displacement of the steering wheel divided by the directional change of the road wheels, may be varied depending upon driving conditions. That is, at low speeds, such as encountered during parking lot maneuvering, a low steering ratio is desired to provide a large directional change in the road wheels while minimizing the steering input. In contrast, at high speeds, a high steering ratio may be desired to provide a slight change in road wheel orientation in response to a turn of the steering wheel. Thus, in steer-by-wire systems, the angular displacement of the steering wheel when the stop is engaged may vary, and it is desired to alert the driver that the road wheel stop has been engaged that is not dependent upon a predetermined angular displacement of the steering wheel.
Therefore, there is a need for a driver interface subsystem that is capable of braking the steering wheel when the road wheels have engaged a stop or are unable of further directional change and thereby alerts the driver in a manner similar to mechanical system with which the driver is familiar.